1. Field of the Invention
The present invention is directed to a radio frequency antenna for a nuclear magnetic resonance tomography apparatus, and in particular to such an antenna having a cylindrical sheath which is radio frequency-tight and low frequency-transmissive which surrounds at least two interior conductors extending parallel to a cylinder axis, the inductance of the inside conductors forming a transmission line resonator in combination with at least one shortening capacitor.
2. Description of Prior Art
Nuclear magnetic resonance tomography devices are used in the field of medical diagnostics, wherein an image similar to an x-ray tomogram is constructed from the three-dimensional spin density of atomic nuclei in an examination subject, or from the distribution of relaxation time of such nuclei, by computational or measuring analysis of integral proton resonance signals. An examination subject, such as a human body, is introduced into a strong, uniform magnetic field, known as the fundamental field, which aligns the nuclear spins subject. Gradient coils are provided which generate selected spatially oriented different magnetic fields. A high-frequency antenna excites the nuclear spins and receives the signal emitted by the excited nuclei. This antenna, which is a radio frequency antenna, is usually connected to a transmitter and to a receiver via matching capacitors, as well as through a transmission/reception diplexer.
A radio frequency antenna is described in European application 0 073 375, corresponding to U.S. Pat. No. 4,506,224, which is formed by a cylindrical sheath which is radio frequency-tight and low frequency-transmissive. The sheath consists of material having a good electrical conductivity, and surrounds at least two interior conductors extending parallel to the longitudinal axis of the cylinder. The interior conductors and the sheath are connected to a transmitter and to a receiver. The interior conductors are terminated with shortening capacitors such that the inductance of the interior conductors in combination with the capacitance of the shortening capacitors form a transmission line resonator. The conductor pairs enable wave propagation inside the sheath at high frequency of, for example, 100 MHz, whereby resonant conditions are established, so that fields which oscillate in equiphase arise in the imaging volume, and standing waves are formed on the interior conductors. The shortening capacitors limit the interior conductors to a length which approximately corresponds to the examination subject, ie., to the size of a human body. In addition to constituting a conductive member for the radio frequency field, the sheath simultaneously functions as a radio frequency shielding from external components. The low-frequency gradient fields can propagate substantially unimpeded in the imaging volume, by contrast, due to the low-frequency permeability of the sheath. The sheath has a thickness which is preferably less than 100 .mu.m. The radio-frequency antenna is dimensioned for one resonant frequency.
The radio frequency antennas are usually provided in magnetic resonance imaging devices for proton imaging. There is, however, an increasing demand that other atoms, for example phosphorous, fluorine or sodium, be investigated in the same system. Because these atomic nuclei have highly different nuclear magnetic resonances, they can only be investigated with a correspondingly broadband radio frequency antenna. Due to the demand for low systems losses, however, a broadband embodiment of a radio frequency antenna can only be produced with an undesirably high outlay. Different radio-frequency antennas have therefore previously been used for investigating respectively different atomic nuclei. Given employment of radio frequency antennas having relatively slight size, for example, surface coils and head coils, these can be interchanged without significant outlay. This is a practical impossibility for whole-body resonators, and consequently these are only utilized for investigating predetermined atoms, for example exclusively for proton experiments.